Guide arrangement for hydraulic cylinders

ABSTRACT

Guide-ring arrangement on a cylindrical guide part for the relative guiding of an elongate guided part having a guide ring where the rear surface projects in a convex manner.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is the national stage of International Application No.PCT/EP95/03874 filed on Sep. 29, 1995.

FIELD OF INVENTION

The present invention relates generally to guides for cylinders and ismore particularly concerned with a guide ring assembly having improvedoperating characteristics.

BACKGROUND OF THE INVENTION

It is known (H. K. Muller: Abdichtung bewegter Maschinenteile [Sealingof moving machine parts], Waiblingen, 1990, p. 190, 191) to provideguide rings in hydraulic cylinders for the flush alignment of the rod.On the one hand, they must be sufficiently dimensioned to absorb theguide forces. On the other hand, they must not damage the very finelyprocessed surface of the rod. Instead of the metal guides previouslyused, guide rings made of materials with a relatively low modulus ofelasticity, for example thermoplastic materials, are thereforepreferred, which can yield more easily in the event of an inclinedposition of the rod relative to the cylinder. However, its strength isno longer adequate to withstand the loading which has increased in thecourse of time. Thermosetting materials are therefore used to a greaterextent, but again experience has proved that damage to the rod surfaceoccurs or the guide rings are destroyed.

The invention is based on an analysis of the damage, which shows thatthe inclined position of the rod relative to the cylinder leads to farhigher loading of the guide ring than was previously assumed. Such aninclined position namely results in pressing of the edges which may leadto a peak load far exceeding ten times the nominal load. Theforce-absorbing capacity of the guide rings also cannot simply beincreased by lengthening them in the axial direction or connecting aplurality of guide rings one behind the other because, in an inclinedposition of the rod, they are load-bearing in any case only on part oftheir length. In contrast, the problem of the edge load in the case oflonger guide rings compared to shorter ones may be further exacerbatedbecause the inclined position of the rod relative to the cylinder causesan additional radial space requirement of the rod at the guide-ringedges, the amount of which is not only proportional to the sine of theinclined position of the rod, but is also proportional to the axiallength of the guide ring. Whenever the guide ring which is built intothe cylinder and interacts with the surface of the rod is oftenmentioned here and in the following text for reasons of simplicity, thisshould also, mutatis mutandis, mean the guide ring which is provided onthe piston and interacts with the cylinder tube.

To relieve a piston or a rod guide from the forces originating from aninclined position of the rod, it is known (Patent Abstracts of JapanVol. 7 No. 183 (M-235) (1328) Aug. 12, 1983--JP-A-58 084 240) to connecta spherical roller bearing between the rod and the piston or between thecylinder and the rod guide. Instead, as is known in another context(FR-A 2 673 449), there could also be spherical mobility of the guidering. However, the production of spherical surfaces involves substantialexpense. This applies even more so because the mutually supportingspherical surfaces have to be of identically spherical design since thependulum motion taking place between them generally takes place underhigh forces and therefore a lack of congruence would lead to a highlevel of wear.

SUMMARY OF THE INVENTION

The invention is based on the object of providing a guide-ringarrangement which permits high guide forces with little loading of thesliding surface and is not subject to the high production expense ofspherical surfaces. The same applies in relation to the guiding of apiston relative to an elongate cylinder tube.

The solution according to the invention consists in providing a guidering of the type described that is held in a plane running perpendicularto the access of the guide part even when an angle of deviation from theaxis occurs so that the guide ring will be twisted.

Starting from the observation that the pressing of the edges of theguide ring increases enormously in the event of an inclined position ofthe rod and in this case the load-bearing surface of the guide ring isreduced at the same time, the solution according to the invention liesin the concept that the regions of the guide ring loaded in each case bythe guide forces of the rod can follow the inclined position of the rod.Since, in this case, the ring remains fixed in its plane and the rodcannot follow in the manner of a ball joint, it becomes twisted initself. This torsion with simultaneous support by the housing is madepossible due to the fact that the rear surface of the guide ring and theassociated supporting surface of the housing project in a convex mannerin their axially central region in relation to one another while,towards the ends, they enclose a gap which increases in width. Inrelation to one another, they are ball-shaped, in which case it ispossible for the ball shape either to be limited to the rear surface ofthe guide ring with a cylindrical supporting surface or to thesupporting surface with a cylindrical rear surface of the guide ring orto be distributed over both surfaces. The ball shape permits the guidering to tilt in an inclined position of the rod and thus to adapt to itwith corresponding twisting. This should not be confused with thecongruently spherical design of the rear surface of the guide ring withthe supporting surface in the sense of spherical pivotability of theguide ring. On the contrary, this is obtained axially, for example byfitting it into a receiving groove so that as a whole it cannot followthe rod by a pivoting movement. It maintains its position in a planeperpendicular to the cylinder axis. The inventive adaptation to theinclined position of the rod is therefore only possible due to the factthat it is deformed differently in sections and is thus twisted inrelation to a circumferential line.

The solution thus appears to be paradoxical since the contact surface ofthe guide ring against the supporting surface of the groove receiving itis reduced by this measure. However, the invention has recognized thefact that this nevertheless involves a significant improvement in theloading conditions. Whereas, with a fixed guide ring, the loading isconcentrated on the edges; in the case of the invention it occursprimarily in the central region of the guide ring, specifically in amanner similar to Hertzian loading. Whereas the loading concentrated onthe edge of the guide ring may lead to the material flowing plasticallyor breaking out there, it easily withstands even higher loading inregions located further towards the inside because the materialundergoing loading is supported on all sides all around.

In principle, any design of the rear surface of the guide ring or of theassociated supporting surface in which these surfaces are supported onone another in their axially central region while they gain spacing fromone another towards the ends is suitable to implement the invention.However, a curvature of one or both of these surfaces which is curvedmonotonically in meridional section is preferred. Good results wereachieved with a curvature following a circular arc, that is to say thecurvature is spherical, the radius of curvature being equal to half thediameter of the rear surface of the guide ring. However, it is usuallymore expedient for the curvature to have a greater radius. Itexpediently follows the formula R=L/(2 sineα), R being the radius of thecurvature, L the axial length of the guide ring and α the maximum angleof deviation from the axis. The radius of the curvature is generally atleast about twice as large as the radius of the guide ring, the radiusof the curvature increasing from the central region of maximumprojection in each case towards the ends according to a particularfeature of the invention. This feature has the advantageous consequencethat the force-transmission conditions become more favourable withincreasing deformation of the guide ring due to an inclined position ofthe rod.

The inventive solution is not only paradoxical due to theforce-transmission conditions on the back of the guide ring, whichappear to be unfavourable at first sight, but also because, owing to theexpected deflection of the guide surface of the guide ring facing therod, a reduction in the load-bearing part of the said guide surface isto be expected. Although this expectation comes about, it does not havethe adverse effects feared. In a similar way to the force transmissionfrom the rear surface of the guide ring to the supporting surface, thedeflection of the guide ring in conjunction with its inclined positionleads to the fact that the principally loaded zone is shifted from theguide-ring edges to regions of the guide surface located more centrally.On the one hand, the guide-ring material is able to withstand higherloading there. On the other hand, this improves the lubricationconditions. This is because the contact pressure which increases fromthe edge of the guide surface to the principally loaded zone, like awedge gap, results in fact in an approximation to hydrodynamiclubrication.

The guide surface of the guide ring may be cylindrical, as is customaryin previously known guide rings. According to a particular feature ofthe invention, however, it is slightly hollowed in a concave manner. Inthis case, the dimensions of the hollow and of the deformationresistance of the guide ring are matched to one another in such a waythat, in that circumferential zone of the guide ring which principallyhas to absorb the lateral guide force, the deformation of the ring hasprogressed, at the latest when the maximum expected guide force occurs,to the extent that the surface of the guided part rests against the baseof the hollow. As a result, on the one hand a better force distributionover the entire axial length of the guide ring in the loaded section isachieved. On the other hand, this results in a scraping effect at theend edges of the guide ring if these are expediently designed with asmaller diameter of the guide surface than corresponds to the diameterof the guided rod. In the opposite case that a guide ring on a pistoninteracts with a cylinder bore, the diameter of the guide surface iscorrespondingly slightly larger at its axial ends than the diameter ofthe cylinder bore. Particles which may have an abrasive effect are thusprevented with a high probability from entering the guide ring. Incontrast, it does not matter if they enter between the rear surface ofthe guide ring and the supporting surface because they become embeddedthere without damage during the rolling movement of the ring. A furtheradvantage of this design consists in the fact that the prestress of theguide ring achieved by the specified dimensioning leads, in the case ofsmall guide forces, to a clearance-free basic position and thus toparticularly small deflections within the spring path of the two limbsof the guide ring (viewed in meridional section). The hydrodynamiclubricating effect mentioned in more detail above is not eliminated bythe hollowed design of the guide surface at high loads at which it isparticularly important.

Surprisingly, the hollow of the guide surface of the guide ring inconjunction with the convex design of its rear surface does notnecessarily lead to reduced guiding quality under a higher load in thesense of a clearance. According to the invention, in fact the largestdiameter of the guide surface in its central region is not designed tobe substantially larger than the dimension resulting if the diameter ofthe rod is increased by the production tolerances and the amount ofthermal expansion (the opposite for use of the guide ring on a pistonfor guiding relative to a cylinder tube). When utilizing all theproduction tolerances and with maximum thermal expansion of the guidering, the radial clearance is zero. The conditions are therefore not atall less favourable than in the case of a guide ring with solelycylindrical guide, rear and supporting surfaces. On the contrary, in thepreviously known cylindrical guide rings, a greater clearance has to beprovided so that the inevitable inclined position of the rod can bepermitted without the guide ring being destroyed solely because of thisinclined position.

It has been described above that the deformation of the guide ring takesplace under torsion relative to a circumferential line. In order tofacilitate this deformation, provision may be made according to theinvention for deformation points of reduced deformation resistance to beprovided distributed over the circumference. The capability of thesepoints to undergo easier deformation relates primarily to the torsionsince successive ring sections will assume a different inclined positionrelative to the cylinder axis. However, other deformations may also beinvolved, for example changes in the spacing in the circumferentialdirection, primarily at the axial ends of the guide ring.

The reduced deformation resistance is expediently brought about byreducing the cross-sectional area in the meridional section where theconnection of integrally designed ring sections is concerned. Thisreduction expediently takes place towards the axial centre of the guidering since the deformations are the smallest there. It is not necessaryfor the deformations to take place elastically. They may also be of aplastic nature. They may even lead to rupture without this impairing thefunction. The deformation points may thus be designed as intendedrupture points. The ring sections which are no longer firmly connectedto one another after the rupture are held in the intended position bythe component in which they are mounted, for example by the end-faceflanks of an assembly groove.

The guide ring does not need to be of integral design; on the contrary,it can be composed of a plurality of non-integral ring sections. For thepurpose of easier assembly, however, these ring sections are expedientlycoupled to one another in such a way that they can be treated as a unit.For example, they may be coupled to one another by a pin and bore toform the finished guide ring.

Also it is not necessary for the guide ring to be closed in thecircumference. On the contrary, it may be formed from a band section, asis known per se. Any divisions should be radial and parallel to the axisso that circumferential stresses do not lead to a displacement of bandends adjoining one another obliquely.

All materials which have sufficient sliding, bearing and strengthproperties can be considered for the design of the guide ring accordingto the invention. Plastics are preferably used. The choice ofthermoplastic or thermosetting materials depends on the respectiveapplication. Bearing metals should also not be ruled out.

Since the specific loading of the guide rings according to the inventionmay lead to high shearing stresses and to substantial tensile stressesin the axial and circumferential direction, it may be expedient to usefiller materials, inserts or composite elements which increase theresistance to this loading. For example, the shearing strength of aplastic can be increased by filler materials. The tensile strength canbe increased by inserting an axial and/or circumferential reinforcement.Finally, it may be expedient to increase the bending resistance of atorque axis running in the circumferential direction, for example byplacing on the back a metal element which is resistant to bending.

BRIEF DESCRIPTION OF THE DRAWING

The invention is explained in greater detail below with reference to thedrawing which illustrates advantageous exemplary embodiments.

FIGS. 1 and 2 show a hydraulic cylinder with a rod extended to adifferent extent;

FIG. 3 shows a diagrammatic partial section through a guide ring withthe rod section located therein;

FIG. 4 shows a cross-section of a guide ring on an enlarged scale; and

FIG. 5 shows a modified embodiment of the cross-section of the guidering.

DESCRIPTION OF A PREFERRED EMBODIMENT

When relatively high transverse forces act on the rods of hydraulicapparatuses, a torque results between the rod 1 and the cylinder 2 whichhas to be transmitted by the interacting guide surfaces of theseelements. This purpose is served by guide rings 3 which are provided onthe piston and on the cylinder opening in order to interact in a gentlemanner in each case with the sensitive, very finely processed surface ofthe rod or the cylinder bore. When--as illustrated by dotted/dashedlines in FIG. 2--the rod is in an inclined position relative to thecylinder, the loading in the known guide rings which are rectangular inmeridional section is concentrated at their edges, which may lead todamage to the guide rings and ultimately also to the sliding surfaceinteracting therewith.

In comparison, the guide ring according to the invention can yield tothe inclined position of the rod, as is demonstrated in FIG. 3.

The guide ring 3 is mounted in the cylinder housing 4 in a groove whosebase 5 forms a supporting surface for the guide ring. It is held in theaxial direction with little clearance by means of the end faces 6 of thegroove, so that it remains aligned with the plane 8 perpendicular to thecylinder axis 7 even in the event of an inclined position of the rod. Ifthe rod 1 is inclined at the angle alpha relative to the cylinder in theplane of the drawing, the annular cross-sections supporting the rod canhowever follow this inclined position of the ring because the rearsurface 9 of the guide ring 3 is not held firmly by the supportingsurface 5. Since the ring is held firmly in the plane perpendicular tothe axis 7, it undergoes torsion.

In the loaded ring section, the entire guide surface 10 rests againstthe surface of the rod 1. This does not mean that all the parts of thissurface are involved to an equal extent in the transmission of the guideforce. On the contrary, the pressing in that region of the guide surfacewhich is located opposite the part 9 resting against the supportingsurface 5 will undergo higher loading while the adjacent regions of theguide surface 10 have to absorb a lower load as a result of deflectionof the guide ring 3. Nevertheless, better load distribution takes placein the guide surface 10 than in the previously known guide rings.Moreover, this type of loading is more favourable because it is similarto Hertzian loading. Although the radial extent of the ring is too smallfor the Hertzian equations to be able to be applied unreservedly, thesurface regions undergoing loading are supported all around by ringmaterial, so that the permissible load is far higher than in the eventof edge loading of the previously known rings.

The loading of the guide ring on the back is to be assessed similarly.Although the curvature of the rear surface leads to a loadconcentration, the fact that the loading is similar to Hertzian loadinghas a favourable effect here, too, so that the specific load-bearingcapacity of the material is far higher than in the known loading of thepreviously known rings.

In view of a favourable force transmission from the rear surface of theguide ring to the supporting surface, the largest possible radius ofcurvature of the rear surface in the meridional plane is desirable. Itis at least large enough for the tangent to the rear surface to enclosean angle beta with the direction of the supporting surface 6 in theremotest region of the guide ring (FIG. 4), which angle is not smallerthan the angle of inclination alpha of the rod. In this case, however,it should be ensured that, in the event of that inclined position of therod and the guide ring in which the maximum loading by the guide forcetakes place, the core of the loading region of the guide ring still hassufficient spacing from its edge. Accordingly, that region of the rearsurface 9 in which the tangent angle beta is equal to the maximum angleof inclination alpha of the rod should have sufficient spacing from theend face of the guide ring.

Since the guide force generally increases with an increasinginclination, it may be expedient to allow the radius of curvature of therear surface to increase gradually from the centre towards the end facesin order thus to provide more favourable Hertzian loading conditions inthe region of the higher loading outside the central region.

In the drawings, it has been assumed that the cross-sectional shape ofthe guide ring is symmetrical in relation to its centre plane 8. This isnot essential. In contrast, it may be expedient for the region of therear surface 9 or of the supporting surface 5 projecting furthest to bearranged off-centre nearer to that end face of the guide ring whichfaces inwards with regard to the overall arrangement of the hydrauliccylinder. In the case of that guide ring which is provided at theopening of a cylinder, this is the end face of the guide ring facing thepiston of the rod. In the case of the guide ring arranged on a pistonand interacting with the cylinder bore, this is the end face which isoriented towards the cylinder opening. This is based on theconsideration that an inclined position of the guide ring on the loadedside always takes place only in one direction, the principal loadingzone of the guide ring migrating outwards with regard to the overallarrangement. Thus if it were to be assumed, for example, that FIG. 4depicts a guide ring which is built into the cylinder and whose end face12 appearing on the right in the drawing faces outwards, it isconceivable to allow the ring to end on the inner side at the plane 13illustrated by dotted/dashed lines, that is to say with a slight spacingfrom the centre plane 8 towards the inner side.

The magnitude of the radial guide forces to be absorbed by the guidering depends on the angle of inclination of the rod relative to thecylinder. Since clearance in the guide ring increases the inclination,it must be kept as small as possible. It should not be greater than thesum of the radial manufacturing tolerances of the three elements,housing, rod and guide ring. This clearance is indicated in FIG. 4between the guide surface 10 and the surface of the rod indicated bydotted/dashed lines.

According to FIG. 5, this clearance can be ruled out--at least for smallguide forces--by the guide surface 10 being hollowed out slightly sothat, in the edge regions 14, it at least reaches, but preferably goesbeyond the surface of the rod, again indicated by dashed lines, evenwith a summation of the largest manufacturing tolerances. The lattercase is illustrated in FIG. 5, where it can be seen that the edgeregions 14 of the guide ring 3 extend radially inwards beyond thesurface line of the rod, indicated by dashed lines, in the unstressedstate. This means that the guide ring must be deformed elasticallyduring assembly. The edge regions 14 then lie on the surface of the rod,while the rear surface 9 is deformed in the direction of the dashed line9'.

As long as the radial guide force transmitted from the guide ring to therod is not higher than the elastic prestressing force of the guide ring,the arrangement is completely without clearance. In the case of higherforces, the edge regions 14 of the guide ring will gradually yieldradially outwards, in which case, depending on the shaping of the guidesurface 10, an increasing proportion thereof may come into contact withthe rod surface. In this case, the contact surface migrates increasinglytowards the centre of the guide ring, as a result of which its elasticproperties become increasingly more rigid until ultimately completecontact of the rod surface against the guide surface 10 takes place evenin its region which was originally furthest away from the rod. Theposition of this region thus determines the maximum clearance. Thecondition that the clearance should not be substantially greater thanthe sum of the manufacturing tolerances and amounts of thermal expansioninvolved is therefore applied in this embodiment to the region of theguide surface which originally has the greatest radial spacing from therod surface.

Although, in the description of the example, reference has principallybeen made to that guide ring which is built into the cylinder andinteracts in a sliding manner with the rod surface, it is obvious thatthese explanations apply correspondingly likewise to that guide ringwhich is installed on the piston or at the end of the plunger. Itscylindrical guide surface interacts with the surface of the bore of thecylinder, while the convexly curved rear surface is supported on thebase of the groove which receives the ring.

What is claimed is:
 1. A guide ring arrangement for a cylindrical guidepart having a central axis for the relative guiding of an elongateguided part having a cylindrical surface comprising a guide ring and aholding device for holding the guide ring, said guide ring having endwalls, an essentially cylindrical guide or sliding surface for slidinginteraction with the cylindrical surface of the guided part and a rearmounted surface spaced from the guide surface by the thickness of thering, said holding device having an essentially cylindrical supportingsurface interacting with the rear surface of the guide ring, saidinteracting rear surface having a convex projection with an apex withina plane perpendicular to said axis of said guide part, said convexprojection being designed as a curvature which is curved monotonicallyin the longitudinal section and corresponds approximately to theformula: R=L/(2 sineα) wherein L is the axial length of the guide-ringand α is the maximum angle of deviation from the axis, said supportingsurface and said rear surface defining a gap therebetween having a widthwhich increases as the distance from the apex increases, the holdingdevice having end wall surfaces at opposite ends of said supportingsurface for engagement by the end walls of the guide ring torestrainably hold the guide ring apex in said plane runningperpendicular to the axis of the guide part even when an angle (α) ofdeviation from the axis occurs between the guide part and the guidedpart whereby the guide ring is deformed in the presence of such an angle(α) of deviation from the axis.
 2. Guide-ring arrangement according toclaim 1, characterized in that the guide surface (10) is hollowed in aconcave manner.
 3. Guide-ring arrangement according to claim 2,characterized in that the dimensions of the hollow and of thedeformation resistance of the guide ring (3) are calculated such thatthe base of the hollow rests against the surface of the guided part atthe latest at the maximum expected guide force.
 4. Guide-ringarrangement according to claim 3, characterized in that the smallestdiameter of the guide surface (10) is smaller at its axial ends than thediameter of the guided part designed as a rod.
 5. Guide-ring arrangementaccording to claim 4, characterized in that the largest diameter of theguide surface (10) is not larger in said plane than the diameter of theguided part designed as a rod plus production tolerances and an amountof thermal expansion.
 6. A guide ring arrangement for a cylindricalguide part having a central axis for the relative guiding of an elongateguided part having a cylindrical surface comprising a guide ring and aholding device for holding the guide ring, said guide ring having endwalls, an essentially cylindrical guide or sliding surface for slidinginteraction with the cylindrical surface of the guided part and a rearmounted surface spaced from the guide surface by the thickness of thering, said holding device having an essentially cylindrical supportingsurface interacting with the rear surface of the guide ring, saidinteracting rear surface having a convex projection with an apex withina plane perpendicular to said axis of said guide part, said convexprojection being designed as a curvature which is curved monotonicallyin the longitudinal section and is at least approximately equal to twicethe radius of the guide ring, said supporting surface and said rearsurface defining a gap therebetween having a width which increases asthe distance from the apex increases, the holding device having end wallsurfaces at opposite ends of said supporting surface for engagement bythe end walls of the guide ring to restrainably hold the guide ring apexin said plane running perpendicular to the axis of the guide part evenwhen an angle (α) of deviation from the axis occurs between the guidepart and the guided part whereby the guide ring is deformed in thepresence of such an angle (α) of deviation from the axis.
 7. A guidering arrangement for a cylindrical guide part having a central axis forthe relative guiding of an elongate guided part having a cylindricalsurface comprising a guide ring and a holding device for holding theguide ring, said guide ring having end walls, an essentially cylindricalguide or sliding surface for sliding interaction with the cylindricalsurface of the guided part and a rear mounted surface spaced from theguide surface by the thickness of the ring, said holding device havingan essentially cylindrical supporting surface interacting with the rearsurface of the guide ring, said interacting rear surface having a convexprojection with an apex within a plane perpendicular to said axis ofsaid guide part, said convex projection being designed as a curvaturewhich is curved monotonically in the longitudinal section and increasesfrom the region of maximum projection towards the ends, said supportingsurface and said rear surface defining a gap therebetween having a widthwhich increases as the distance from the apex increases, the holdingdevice having end wall surfaces at opposite ends of said supportingsurface for engagement by the end walls of the guide ring torestrainably hold the guide ring apex in said plane runningperpendicular to the axis of the guide part even when an angle (α) ofdeviation from the axis occurs between the guide part and the guidedpart whereby the guide ring is deformed in the presence of such an angle(α) of deviation from the axis.